The caveat to the expense point is "in the US" Good NAs are rising in value here where NBs are still heading down, and supply of NBs is better too. You can get a decent NA for about a grand (mine was around that) and an NB for about £1500, which isn't too bad a price difference.
I've always thought the used market for Miatas in the States seemed unrealistically inflated. We may get screwed on new prices in the UK but we have a decent used market to console us. As long as rust is avoided.
Aircon is great but in the UK it's something I'd not bother with on MX5s. Our weather never really gets hot enough where dropping the roof wouldn't be a better option than turning on the aircon. I know that's not really the case in some places over there.
It's why convertibles sell better in the UK than anywhere else in Europe - although our weather is crap a lot of the time, when it does get sunny it's usually perfect convertible weather. Oh, and the heater is excellent in MX5s so even when it's freezing outside you can still have the top down.